Device for fault localization in repeaterless transmission system

ABSTRACT

A device is used for receiving input signal light and pump light from a first remote site through a first optical fiber and a second optical fiber, respectively, and for sending amplified signal light through a third optical fiber to a second remote site. The device includes an amplifier unit for amplifying the input signal light by using the pump light to transmit the amplified signal light to the third optical fiber, the amplifier unit having a prevention unit for preventing light coming back through the third optical fiber from entering the amplifier unit, and a path providing unit for providing a path inside the device, the path leading reflection light coming through the third optical fiber to a connection point of the device, the connection point having optical connection with the first remote site, wherein the reflection light is a light pulse for fault localization reflected at a fault point on the third optical fiber, the light pulse for fault localization sent to the third optical fiber from the first remote site via the device.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to optical-fiber communication,and particularly relates to remote amplifiers used in a repeaterlesstransmission system based on optical fibers and to a method oflocalizing fault points on optical fibers.

2. Description of the Related Art

A transmission system using optical fibers is classified into a transittransmission system and a repeaterless transmission system. The transittransmission system uses transit trunks provided at equal intervals on atransmission line which connect between two terminal stations, and therepeaterless transmission system does not use any transit trunk on thetransmission line. Taking an example of ocean-bottom optical-fibertransmission, the transit transmission system is used for long-distanceand deep-sea transmission, and the repeaterless transmission system isused for short-distance and shallow-sea transmission.

FIG. 1 is an illustrative drawing showing a schematic configuration ofthe repeaterless transmission system. Two terminal stations 1 areconnected with each other via an optical-fiber cable 3. Each of theterminal stations 1 includes an optical-signal sending unit 101 forsending an optical signal and an optical-signal receiving unit 111 forreceiving an optical signal.

The repeaterless transmission system is relatively inexpensive becausetrunks and power supply for the trunks are not necessary, but has alimit on a transmission distance. In order to obviate this problem, oneor more remote amplifiers are provided on the transmission line. Aremote amplifier receives a pump light from a terminal station, andamplifies an optical signal to lengthen the transmission distance. Thisis called a remote pump method.

FIG. 2 is an illustrative drawing showing a detailed configuration ofthe repeaterless transmission system. Two remote amplifiers 2 areprovided on an optical-fiber cable 6 between the terminal stations 1 atpoints where the pump lights from the terminal stations 1 can reach (thedistance which the pump light can travel is about 100 km). In theconfiguration in which the remote amplifiers 2 are used one each for thetwo terminal stations 1, the distance between the terminal stations 1can be as long as 400 km by setting the remote amplifiers 200 km fromeach other.

In FIG. 2, the terminal station 1 includes a sending unit 10 and areceiving unit 11, and the remote amplifier 2 includes a post amplifier20 and a pre-amplifier 21. Viewed from the terminal station 1 on theleft of the figure, the sending unit 10 and the post amplifier 20constitutes a sending system, and the receiving unit 11 and thepre-amplifier 21 constitutes a receiving system. The post amplifier 20amplifies and optical signal sent from the sending unit 10, and thepre-amplifier 21 enhances the reception sensitivity of the receivingunit 11.

The sending unit 10 includes an optical-signal sending unit 101, a laserdiode 102, a laser diode 103, and a WDM (wavelength divisionmultiplexing) coupler 104. The receiving unit 11 includes anoptical-signal receiving unit 111 and a laser diode 112.

The post amplifier 20 includes a fiber amplifier 201, a WDM coupler 202,and an optical isolator 203. The pre-amplifier 21 includes a fiberamplifier 211, a WDM coupler 212, and an optical isolator 213.

The optical-signal sending unit 101 sends signal light having awavelength of λ₀, and the optical-signal receiving unit 111 receivessignal light having a wavelength of λ₀. The fiber amplifiers 201 and 211are erbium-doped-optical-fiber amplifiers which are excited by pumplight having a wavelength of λ₁. The laser diodes 102 and 103 areexcitation-light sources for effecting front excitation and rearexcitation, respectively, of the fiber amplifier 201. The WDM couplers104, 202, and 212 are optical couplers for splitting or mixing the lightof wavelength λ₀ and the light of wavelength λ₁. The optical isolators203 and 213 are devices which allow a one-way passage of light as shownby an arrow in the figure.

In a normal operation, the sending system operates as follows.

The signal light (wavelength λ₀) from the optical-signal sending unit101 and front-pump light (wavelength λ₁) from the laser diode 102 aremixed in a sending-purpose optical fiber 6a by the WDM coupler 104. Themixed light is transmitted via the sending-purpose optical fiber 6a toan input end of the fiber amplifier 201 of the post amplifier 20.Rear-pump light (wavelength λ₁) from the laser diode 103 is transmittedvia an excitation-purpose optical fiber 6c to the post amplifier 20,and, then, is joined with an output end of the fiber amplifier 201 bythe WDM coupler 202. The signal light amplified by the fiber amplifier201 passes through the WDM coupler 202 and the optical isolator 203,and, then, is transmitted to a sending-purpose optical fiber 6a' towardthe terminal station 1 on the right of the figure.

The receiving system operates as follows.

Rear-pump light (wavelength λ₁) from the laser diode 112 is transmittedvia an excitation-purpose optical fiber 6d to the pre-amplifier 21, andis joined with an output end of the fiber amplifier 211 by the WDMcoupler 212. The signal light supplied to the fiber amplifier 211 from areceiving-purpose optical fiber 6b' is amplified by the fiber amplifier211, and passes through the WDM coupler 212 and the optical isolator213. Then, the signal light is transmitted via a receiving-purposeoptical fiber 6b to the optical-signal receiving unit 111.

When the signal light coming out of the fiber amplifier 201 or the fiberamplifier 211 enters an optical fiber for transmission, Rayleighscattering causes backward scattering of the signal light. If thisbackwardly scattered light comes back to enter the fiber amplifier, thescattered light is amplified and transmitted to the optical fiber again,thereby causing a deterioration of the signal-to-noise ratio of thesignal light. The optical isolators 203 and 213 are provided at theoutput end of the fiber amplifiers 201 and 211, respectively, to preventthe entering of the backwardly scattered light.

When a fault of an optical-fiber cable such as a cut-off of the cable,an unexpected loss of the optical signal on the cable, or the likehappens in the optical-fiber transmission system, a fault point on thecable needs to be identified by the terminal stations. In therepeaterless transmission system, since no power supply line is providedto remote amplifiers, the localization of the fault point needs to bedone by using the optical fibers. A fault of an optical-fiber cable canbe caused by a cut-off of the entire cable or a cut-off of some of theoptical fibers inside the cable. Therefore, all the optical fibers inthe cable must be individually checked for localizing the fault point.

The localization of the fault point for each optical fiber can be doneby using an OTDR (optical time domain reflectometer). In thelocalization of the fault point based on the OTDR, a light pulse forfault localization is input from a terminal station into an opticalfiber to be tested. The optical pulse reaching the fault point isreflected and scattered by the fault point to come back to the terminalstation. At the terminal station, the reflected and scattered light isdetected. An analysis of the detected light gives an evaluation todecide the position of the fault.

FIGS. 3A and 3B are illustrative drawings showing schematicconfigurations of two different types of the OTDR. An OTDR 300 of FIG.3A includes an analysis/display unit 301 for analyzing a returned signaland displaying the result and a sending/receiving unit 302. The OTDR 300is of a type having an optical-pulse sending unit and a receiving unitwhich are integrated as one unit. The sending/receiving unit 302 sends alight pulse for fault localization to an optical fiber 303, and receivesreflected and scattered light from a fault point 350. The OTDR 300 asshown in FIG. 3A has a dynamic range of about 40 dB, and is capable oflocalizing a fault point as far as 200 km from the terminal station.

An OTDR 310 of FIG. 3B includes an analysis/display unit 311, a sendingunit 312, and a receiving unit 313. The OTDR 310 is of a type which hasan optical-pulse sending unit and a receiving unit as separate units.The sending unit 312 sends a light pulse for fault localization to anoptical fiber 314, and the receiving unit 313 receives reflected andscattered light from a fault point 350 via another optical fiber 315.Here, a transit trunk 320 having photocouplers CPL is provided. The OTDR310 can be used in the transit transmission system, and is capable ofdetecting a fault point between transit trunks. The OTDR 310 as shown inFIG. 3B has a dynamic range of about 20 dB, and is capable of localizinga fault point as far as 100 km from the nearest transit trunk.

In the repeaterless transmission system of the related art as shown inFIG. 2, fault localization for the optical-fiber cable is easily carriedout by using the OTDR, as long as the fault point is located between aterminal station and a remote amplifier. When the fault point is presentbetween remote amplifiers, however, a terminal station should detect thefault point via a remote amplifier. In this case, the optical pulse fromthe OTDR of the terminal station can reach the fault point via anoptical isolator of the remote amplifier because the direction of theoptical-pulse transmission corresponds to the direction of signaltransmission. However, reflected and scattered light coming back fromthe fault point is prevented from passing through the optical isolatorbecause the direction of the returning light is opposite of thedirection of signal transmission. Thus, an OTDR cannot be used for faultlocalization for optical fibers in the prior art system.

Accordingly, there is a need in the repeaterless transmission system fora remote amplifier and a method of fault localization which allows faultlocalization using an OTDR at a terminal station to detect a fault pointeven when the fault point is located beyond the remote amplifier.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to providea remote amplifier and a method of fault localization which satisfy theneed described above.

It is another and more specific object of the present invention toprovide a remote amplifier and a method of fault localization whichallows fault localization using an OTDR at a terminal station to detecta fault point even when the fault point is located beyond the remoteamplifier.

In order to achieve the above objects, a device according to the presentinvention is used for receiving input signal light and pump light from afirst remote site through a first optical fiber and a second opticalfiber, respectively, and for sending amplified signal light through athird optical fiber to a second remote site. The device includes anamplifier unit for amplifying the input signal light by using the pumplight to transmit the amplified signal light to the third optical fiber,the amplifier unit having prevention unit for preventing light comingback through the third optical fiber from entering the amplifier unit,and a path providing unit for providing a path inside the device, thepath leading reflection light coming through the third optical fiber toa connection point of the device, the connection point having opticalconnection with the first remote site, wherein the reflection light is alight pulse for fault localization reflected at a fault point on thethird optical fiber, the light pulse for fault localization sent to thethird optical fiber from the first remote site via the device.

According to the device described above, the path is provided inside theremote amplifier to lead the reflection light from the third opticalfiber to the connection point which has an optical connection with thefirst remote site. Therefore, upon transmission of the light pulse forfault localization from the first remote site, the light pulse for faultlocalization is reflected at the fault point on the third optical fiber,and can take the path to come back to the first remote site. Thisachieves the localization of the fault point at the terminal station byusing the OTDR even when the fault point is located beyond the remoteamplifier.

Other objects and further features of the present invention will beapparent from the following detailed description when read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustrative drawing showing a schematic configuration of arepeaterless transmission system;

FIG. 2 is an illustrative drawing showing a detailed configuration ofthe repeaterless transmission system;

FIGS. 3A and 3B are illustrative drawings showing schematicconfigurations of two different types of OTDRs;

FIG. 4 is a block diagram of a remote amplifier according to a firstprinciple of the present invention;

FIG. 5 is a block diagram of a remote amplifier according to a secondprinciple of the present invention;

FIG. 6 is a block diagram of a remote amplifier according to a thirdprinciple of the present invention;

FIG. 7 is a block diagram of a remote amplifier according to a fourthprinciple of the present invention;

FIG. 8 is a block diagram of a remote amplifier according to a fifthprinciple of the present invention;

FIG. 9 is an illustrative drawing showing a first embodiment of arepeaterless transmission system according to the present invention;

FIG. 10 is an illustrative drawing showing a second embodiment of arepeaterless transmission system according to the present invention;

FIG. 11 is a variation of the second embodiment shown in FIG. 10;

FIG. 12 is an illustrative drawing showing a third embodiment of arepeaterless transmission system according to the present invention;

FIGS. 13A and 13B are illustrative drawings showing configurations inwhich a detour route without amplification is provided;

FIG. 14 is an illustrative drawing showing a fourth embodiment of arepeaterless transmission system according to the present invention;

FIGS. 15A and 15B are illustrative drawings showing configurations inwhich a WDM coupler is used in place of an optical coupler of FIG. 14;

FIGS. 16A and 16B are illustrative drawings showing variations of thefourth embodiment of the present invention;

FIG. 17 is an illustrative drawing showing a fifth embodiment of arepeaterless transmission system according to the present invention;

FIG. 18 is an illustrative drawing showing a sixth embodiment of arepeaterless transmission system according to the present invention; and

FIG. 19 is an illustrative drawing showing ranges where a fault-pointcan be identified.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, principles and embodiments of the present inventionwill be described with reference to the accompanying drawings.

FIG. 4 is a block diagram of a remote amplifier according to a firstprinciple of the present invention. In FIG. 4, the same elements asthose of FIG. 2 are referred to by the same numerals, and a descriptionthereof will be omitted.

A remote amplifier 7 of FIG. 4 includes a fault-point-localization node5a, a sending unit 8, a receiving unit 9, and areflected/scattered-light branching unit 31. Thefault-point-localization node 5a is connected to a localization-purposetransmission line 5 extending from the terminal station 1. Thereflected/scattered-light branching unit 31 receives reflected andscattered light coming back to the remote amplifier 7 when afault-point-localization signal is reflected and scattered by a faultpoint 350. The reflected/scattered-light branching unit 31 then leadsthe reflected and scattered light to the fault-point-localization node5a. Here, the sending unit 8 and the receiving unit 9 are equivalent tothe post amplifier 20 and the pre-amplifier 21 of FIG. 2, respectively.

Operations are as follows. The terminal station 1 sends thefault-point-localization signal to the localization-purpose transmissionline 5. The fault-point-localization signal is then led to thesending-purpose optical fiber 6a' provided on the output side of thesending unit 8 by the reflected/scattered-light branching unit 31. Whenreaching the fault point 350 on the sending-purpose optical fiber 6a',the fault-point-localization signal is reflected and scattered by thefault point 350 to come back to the remote amplifier 7. The reflectedand scattered light is led to the fault-point-localization node 5a bythe reflected/scattered-light branching unit 31. The terminal station 1detects the reflected and scattered light traveling through thelocalization-purpose transmission line 5 to localize the fault point 350on the sending-purpose optical fiber 6a'.

In this method of localizing the fault point, the terminal station 1 maysend the fault-point-localization signal to the sending-purpose opticalfiber 6a rather than to the localization-purpose transmission line 5. Inthis case, the fault-point-localization signal is amplified by thesending unit 8, so that the fault localization is effective for a longerdistance. Return paths for the reflected and scattered light are thesame as for the previous case.

FIG. 5 is a block diagram of a remote amplifier according to a secondprinciple of the present invention. In FIG. 5, the same elements asthose of FIG. 4 are referred to by the same numerals, and a descriptionthereof will be omitted.

A remote amplifier 7A of FIG. 5 includes a sending unit 8A, thereceiving unit 9, and a reflected/scattered-light branching/joining unit32. In FIG. 5, the reflected/scattered-light branching/joining unit 32receives reflected and scattered light coming back to the remoteamplifier 7A when the fault-point-localization signal is reflected andscattered by the fault point 350. The reflected/scattered-lightbranching/joining unit 32 then leads the reflected and scattered lightto the excitation-purpose optical fiber 6c.

Operations are as follows. The terminal station 1 sends thefault-point-localization signal to the excitation-purpose optical fiber6c. Here, the fault-point-localization signal has a wavelength differentfrom that of the pump light. The fault-point-localization signal is thenled to the sending-purpose optical fiber 6a' provided on the output sideof the sending unit 8A by the reflected/scattered-lightbranching/joining unit 32. When reaching the fault point 350 on thesending-purpose optical fiber 6a', the fault-point-localization signalis reflected and scattered by the fault point 350 to come back to theremote amplifier 7A. The reflected and scattered light is led to theexcitation-purpose optical fiber 6c by the reflected/scattered-lightbranching/joining unit 32. The terminal station 1 detects the reflectedand scattered light traveling through the excitation-purpose opticalfiber 6c to localize the fault point 350 on the sending-purpose opticalfiber 6a'.

In this method of localizing the fault point, the terminal station 1 maysend the fault-point-localization signal to the sending-purpose opticalfiber 6a rather than to the excitation-purpose optical fiber 6c. In thiscase, the fault-point-localization signal is amplified by the sendingunit 8A, so that the fault localization is effective for a longerdistance. Return paths for the reflected and scattered light are thesame as for the previous case.

FIG. 6 is a block diagram of a remote amplifier according to a thirdprinciple of the present invention. In FIG. 6, the same elements asthose of FIG. 4 are referred to by the same numerals, and a descriptionthereof will be omitted.

A remote amplifier 7B of FIG. 6 includes the sending unit 8, thereceiving unit 9, and a reflected/scattered-light detouring unit 33. InFIG. 6, the reflected/scattered-light detouring unit 33 receivesreflected and scattered light coming back to the remote amplifier 7Bwhen the fault-point-localization signal is reflected and scattered bythe fault point 350. The reflected/scattered-light detouring unit 33then leads the reflected and scattered light to the sending-purposeoptical fiber 6a by getting around the sending unit 8.

Operations are as follows. The terminal station 1 sends thefault-point-localization signal to the sending-purpose optical fiber 6a.The fault-point-localization signal is then amplified by the sendingunit 8 and transmitted to the sending-purpose optical fiber 6a'. Whenreaching the fault point 350 on the sending-purpose optical fiber 6a',the fault-point-localization signal is reflected and scattered by thefault point 350 to come back to the remote amplifier 7B. The reflectedand scattered light is led to the sending-purpose optical fiber 6a bythe reflected/scattered-light detouring unit 33 getting around(bypassing) the optical isolator of the sending unit 8. The terminalstation 1 detects the reflected and scattered light traveling throughthe sending-purpose optical fiber 6a to localize the fault point 350 onthe sending-purpose optical fiber 6a'.

In this configuration, the reflected/scattered-light detouring unit 33may include an optical amplifier to amplify the reflected and scatteredlight. In this case, the fault localization is effective for a longerdistance.

FIG. 7 is a block diagram of a remote amplifier according to a fourthprinciple of the present invention. In FIG. 7, the same elements asthose of FIG. 4 are referred to by the same numerals, and a descriptionthereof will be omitted.

A remote amplifier 7C of FIG. 7 includes the sending unit 8, thereceiving unit 9, and a reflected/scattered-light branching/joining unit34. In FIG. 7, the reflected/scattered-light branching/joining unit 34receives reflected and scattered light coming back to the remoteamplifier 7C when the fault-point-localization signal is reflected andscattered by the fault point 350. The reflected/scattered-lightbranching/joining unit 34 then leads the reflected and scattered lightto an input node or an output node of the receiving unit 9, so that thereflected and scattered light can reach the terminal station 1.

Operations are as follows. The terminal station 1 sends thefault-point-localization signal to the sending-purpose optical fiber 6a.The fault-point-localization signal is then amplified by the sendingunit 8 and transmitted to the sending-purpose optical fiber 6a'. Whenreaching the fault point 350 on the sending-purpose optical fiber 6a',the fault-point-localization signal is reflected and scattered by thefault point 350 to come back to the remote amplifier 7C. The reflectedand scattered light is led to the input node or the output node of thereceiving unit 9 by the reflected/scattered-light branching/joining unit34. The terminal station 1 detects the reflected and scattered lighttraveling through the receiving-purpose optical fiber 6b to localize thefault point 350 on the sending-purpose optical fiber 6a'.

FIG. 8 is a block diagram of a remote amplifier according to a fifthprinciple of the present invention. In FIG. 8, the same elements asthose of FIG. 4 are referred to by the same numerals, and a descriptionthereof will be omitted.

A remote amplifier 7D of FIG. 8 includes a fault-point-localization node5b, the sending unit 8, the receiving unit 9, and areflected/scattered-light joining unit 35. The fault-point-localizationnode 5b is connected to the localization-purpose transmission line 5extending from the terminal station 1. The fault-point-localizationsignal sent to the localization-purpose transmission line 5 from theterminal station 1 is input to the fault-point-localization node 5b,and, then, is led to the receiving-purpose optical fiber 6b' on theinput side of the receiving unit 9 by the reflected/scattered-lightjoining unit 35. The reflected/scattered-light joining unit 35 receivesreflected and scattered light coming back to the remote amplifier 7Dwhen the fault-point-localization signal is reflected and scattered bythe fault point 350. The reflected/scattered-light joining unit 35 thenleads the reflected and scattered light to an input node of thereceiving unit 9, so that the reflected and scattered light can reachthe terminal station 1.

Operations are as follows. The terminal station 1 sends thefault-point-localization signal to the localization-purpose transmissionline 5. The fault-point-localization signal is input to thereflected/scattered-light joining unit 35 at thefault-point-localization node 5b, whereupon the fault-point-localizationsignal is led to the receiving-purpose optical fiber 6b'. Upon reachingthe fault point 350 on the receiving-purpose optical fiber 6b', thefault-point-localization signal is reflected and scattered by the faultpoint 350 to come back to the remote amplifier 7D. The reflected andscattered light is input to the receiving unit 9 via thereflected/scattered-light joining unit 35. The receiving unit 9amplifies the reflected and scattered signal to send it to thereceiving-purpose optical fiber 6b. The terminal station 1 detects thereflected and scattered light traveling through the receiving-purposeoptical fiber 6b to localize the fault point 350 on thereceiving-purpose optical fiber 6b'.

The remote amplifier 7 of the fifth principle shown in FIG. 8 mayfurther include a fault-point-localization-signal amplifier whichamplifies the fault-point-localization signal received at thefault-point-localization node 5b. This configuration extends aneffective distance range of the fault localization.

In the following, embodiments of the present invention will be describedwith reference to the accompanying drawings. In these embodimentsprovided below, the present invention will be applied to therepeaterless communication system of the related art shown in FIG. 2. Inthese embodiments, operations under fault-free conditions are almost thesame as those of the related art. In the following figures, the sameelements as those of FIG. 2 are referred to by the same numerals, and adescription thereof will be omitted.

FIG. 9 is an illustrative drawing showing a first embodiment of arepeaterless transmission system according to the present invention. Inthe first embodiment, a localization-purpose optical fiber 6e isprovided between the terminal station 1 and a remote amplifier 2A, andan optical coupler 204 is used for joining the localization-purposeoptical fiber 6e with the sending-purpose optical fiber 6a'.

In the normal operation mode, i.e., under fault-free conditions, thelocalization-purpose optical fiber 6e is not used. When a fault pointneeds to be identified, the OTDR 310 of FIG. 3B is used in localizationoperations as follows.

In the terminal station 1, the sending unit 312 of the OTDR 310 isinstalled in place of the optical-signal sending unit 101, and thereceiving unit 313 of the OTDR 310 is connected to a node 107 of thelocalization-purpose optical fiber 6e. The sending unit 312 transmits alight pulse for fault localization having the same wavelength λ₀ as theoptical signal. The fiber amplifier 201 is in an excited condition as inthe normal operation mode, so that the light pulse for faultlocalization is amplified by the fiber amplifier 201 before beingtransmitted to the sending-purpose optical fiber 6a'. Reflected andscattered light coming back from the fault point 350 on thesending-purpose optical fiber 6a' is input to the optical coupler 204 ofthe remote amplifier 2A. The optical coupler 204 leads the reflected andscattered light to the localization-purpose optical fiber 6e, so thatthe receiving unit 313 of the OTDR 310 can detect the reflected andscattered light.

Alternately, the OTDR 300 of FIG. 3A may be used in localizationoperations as follows.

In the terminal station 1, the sending/receiving unit 302 of the OTDR300 is connected to the node 107 of the localization-purpose opticalfiber 6e. A light pulse for fault localization transmitted from thesending/receiving unit 302 reaches the fault point 350 via thelocalization-purpose optical fiber 6e, the optical coupler 204, and thesending-purpose optical fiber 6a'. The reflected and scattered lightcomes back to the sending/receiving unit 302 along the same route in areverse direction.

FIG. 10 is an illustrative drawing showing a second embodiment of arepeaterless transmission system according to the present invention. Inthe second embodiment, an optical coupler 206 is provided at one end ofthe excitation-purpose optical fiber 6c in the remote amplifier 2B, anda band-pass filter 205 connects the optical coupler 206 with the opticalcoupler 204. The band-pass filter 205 has such frequency characteristicsthat it passes only a light pulse for fault localization having adifferent wavelength from that of the pump light. In this embodiment,the light pulse for fault localization has the same wavelength as thatof the light signal.

In the normal operation mode, the pump light traveling through theexcitation-purpose optical fiber 6c is blocked by the band-pass filter205 so as not to reach the optical coupler 204. Thus, there is noadverse effect on the signal light passing through the optical coupler204 toward the sending-purpose optical fiber 6a'. At the time of thefault localization, the OTDR 300 of FIG. 3A is used as follows.

In the terminal station 1, the sending/receiving unit 302 of the OTDR300 is installed in place of the laser diode 103 for the excitationpurpose. The optical-signal sending unit 101 and the laser diode 102 areinactive. The sending/receiving unit 302 transmits a light pulse forfault localization having the same wavelength λ₀ as the signal light.The light pulse for fault localization travels through theexcitation-purpose optical fiber 6c to reach the remote amplifier 2B.Then, the light pulse for fault localization passes through the opticalcoupler 206 and the band-pass filter 205 to reach the optical coupler204, where the light pulse for fault localization is joined to thesending-purpose optical fiber 6a'. The reflected and scattered lightfrom the fault point 350 on the sending-purpose optical fiber 6a' comesback to the terminal station 1 along the same route as described above,and the sending/receiving unit 302 detects the reflected and scatteredlight.

FIG. 11 is a variation of the second embodiment shown in FIG. 10. In aremote amplifier 2B' of FIG. 11, a WDM coupler 2061 is used in place ofthe band-pass filter 205 and the optical coupler 206. The WDM coupler2061 is capable of mixing and splitting light of the wavelength λ₀ andlight of the wavelength λ₁.

In the normal operation mode, the pump light of the wavelength λ₁supplied to the WDM coupler 2061 through the excitation-purpose opticalfiber 6c is split by the WDM coupler 2061 to be entirely directed to theWDM coupler 202. Thus, this pump light has no adverse effect on thesignal light passing through the optical coupler 204. At the time of thefault localization, the OTDR 310 of FIG. 3B is used as follows.

In the terminal station 1, the sending unit 312 of the OTDR 310 replacesthe optical-signal sending unit 101, and the receiving unit 313 of theOTDR 310 replaces the laser diode 103 for the rear-excitation purpose.The sending unit 312 transmits a light pulse for fault localizationhaving the same wavelength as the signal light to the sending-purposeoptical fiber 6a. The fiber amplifier 201 is in an excited condition asin the normal operation mode, so that the light pulse for faultlocalization is amplified by the fiber amplifier 201 before beingtransmitted to the sending-purpose optical fiber 6a'. Reflected andscattered light coming back from a fault point on the sending-purposeoptical fiber 6a' is input to the optical coupler 204 of the remoteamplifier 2B'. The optical coupler 204 splits the reflected andscattered light to direct it to the excitation-purpose optical fiber 6cvia the WDM coupler 2061. The receiving unit 313 of the OTDR 310 candetect the reflected and scattered light at the terminal station 1.

FIG. 12 is an illustrative drawing showing a third embodiment of arepeaterless transmission system according to the present invention. Inthis embodiment, a detour route (j) is provided in parallel to the routealong which the signal light passes, so that a light pulse for faultlocalization can get around the optical isolator 203 by taking thisdetour route.

A remote amplifier 2C of FIG. 12 includes the pre-amplifier 21, thefiber amplifier 201, the WDM coupler 202, the optical isolator 203, theoptical coupler 204, the WDM coupler 2061, a fiber amplifier 207, a WDMcoupler 208, and a circulator 209. The detour route is comprised of thefiber amplifier 207 and the WDM coupler 208, and is connected inparallel to the route for the signal light by the circulator 209 and theoptical coupler 204. The fiber amplifier 207 is used for amplifying thereflected and scattered light along the detour route. In order to supplya pump light to the fiber amplifier 207, a laser diode 105 is providedin the terminal station 1, and a WDM coupler 106 is also provided tocouple the light from the laser diode 103 and the light from the laserdiode 105.

The laser diode 105 transmits a pump light having a wavelength λ₂ usedfor the excitation of the fiber amplifier 207. The WDM coupler 106 mixesthe pump light (wavelength λ₁) from the laser diode 103 with the pumplight (wavelength λ₂) from the laser diode 105, and transmits the mixedlight on the excitation-purpose optical fiber 6c. The WDM coupler 2061splits the two pump lights sent through the excitation-purpose opticalfiber 6c. The WDM coupler 2061 feeds the pump light of the wavelength λ₁to the WDM coupler 202, and feeds the pump light of the wavelength λ₂ tothe WDM coupler 208. The WDM coupler 208 provides the pump light of thewavelength λ₂ for the detour route (j). The circulator 209 directs thereflected and scattered light, which has traveled along the detourroute, into the sending-purpose optical fiber 6a. Also, the circulator209 directs the signal light, which is sent from the terminal station 1through the sending-purpose optical fiber 6a, to the signal-light path(i.e., to the fiber amplifier 201).

In the normal operation mode, the laser diode 105 of the terminalstation 1 sends no pump light to excite the WDM coupler 208. In thiscase, the detour route is provided with attenuation larger than almost20 dB. During the time of fault localization, the OTDR 300 of FIG. 3A isused as follows.

In the terminal station 1, the optical-signal sending unit 101 isreplaced by the sending/receiving unit 302 of the OTDR 300. The laserdiode 105 sends the pump light to put the fiber amplifier 207 in anexcitation state. The sending/receiving unit 302 of the OTDR 300transmits a light pulse for fault localization having the samewavelength λ₀ as the signal light. The light pulse for faultlocalization is amplified by the fiber amplifier 201, and transmitted tothe sending-purpose optical fiber 6a'. The reflected and scattered lightcoming back from the fault point 350 of the sending-purpose opticalfiber 6a' is branched by the optical coupler 204 to travel along thedetour route. The reflected and scattered light is amplified by thefiber amplifier 207, and is directed to the sending-purpose opticalfiber 6a by the circulator 209. Finally, the sending/receiving unit 302of the OTDR 300 in the terminal station 1 receives the reflected andscattered light.

In the third embodiment of FIG. 12, the reflected and scattered light isamplified along the detour route. However, the reflected and scatteredlight may be merely detoured along the detour route to get around theoptical isolator 203 without any amplification.

FIGS. 13A and 13B are illustrative drawings showing configurations inwhich a detour route without amplification is provided. In theconfigurations of FIG. 13A and FIG. 13B, the signal light and the lightpulse for fault localization have different wavelengths.

In FIG. 13A, a signal light (wavelength λ₀) passes through the fiberamplifier 201 and the optical isolator 203, whereas a light pulse forfault localization is led to the detour route because it has awavelength λ₁. Also, reflected and scattered light (wavelength λ₁) isled to the detour route on its return. These selections of routes basedon the wavelengths are achieved by a WDM coupler 2091 and a WDM coupler2041. In this configuration, the detour route has no effect on thesignal light because of the wavelength-dependent route selection.

In FIG. 13B, a detour route is provided to get around only the opticalisolator 203, so that the fiber amplifier 201 is effective for both thesignal light and the light pulse for fault localization. In thisconfiguration, an effective range of the fault localization is extendedcompared to the configuration of FIG. 13A.

FIG. 14 is an illustrative drawing showing a fourth embodiment of arepeaterless transmission system according to the present invention. Inthis embodiment, reflected and scattered light is branched off from asignal-light sending path, and is led to a signal-light receiving path.A remote amplifier 2D of FIG. 14 includes the fiber amplifier 201, theWDM coupler 202, the optical isolator 203, the optical coupler 204, thefiber amplifier 211, the WDM coupler 212, the optical isolator 213, andan optical coupler 214. The optical coupler 204 provides a branchdiverging from the signal-light sending path, and the optical coupler214 connects the branch to the signal-light receiving path. As shown inFIG. 14, a position of the optical coupler 214 can be on an output sideof the pre-amplifier 21 or on an input side of the pre-amplifier 21. InFIG. 14, a first branch connected to the output side of thepre-amplifier 21 is denoted as (f), and a second branch connected to theinput side of pre-amplifier 21 is denoted as (g).

During the time of the fault localization, the OTDR 310 of FIG. 3B isused as follows.

In the terminal station 1, the sending unit 312 of the OTDR 310 isinstalled in place of the optical-signal sending unit 101, and thereceiving unit 313 of the OTDR 310 replaces the optical-signal receivingunit 111. The sending unit 312 of the OTDR 310 transmits a light pulsefor fault localization having the same wavelength λ_(o) as the signallight. The light pulse for fault localization is amplified by the fiberamplifier 201 before being transmitted to the sending-purpose opticalfiber 6a'. The reflected and scattered light coming back from the faultpoint 350 of the sending-purpose optical fiber 6a' is received by theoptical coupler 204. The optical coupler 204 directs the reflected andscattered light to the first branch (f) or the second branch (g), sothat the reflected and scattered light can reach the terminal station 1.The receiving unit 313 of the OTDR 300 in the terminal station 1 detectsthe reflected and scattered light.

When the second branch (g) is used, the reflected and scattered light isamplified by the fiber amplifier 211. Thus, the use of the second branch(g) provides a longer range for the fault localization.

When optical couplers are used for establishing the branch as in thisembodiment, the receiving signal light can be affected bybackwardly-scattered light of the sending signal light during the normaloperation. This is because the backwardly-scattered light travelsthrough the branch to enter the receiving-purpose optical fiber 6b. Inorder to avoid this, losses along the branch should be substantiallylarger than losses of the signal light traveling along the signal-lightpaths.

FIGS. 15A and 15B are illustrative drawings showing configurations inwhich a WDM coupler 2141 is used in place of the optical coupler 214 ofFIG. 14. In these figures, the WDM coupler 2141 replacing the opticalcoupler 214 makes it possible to use a small-loss branch without havingan adverse effect on the receiving signal light. In this case, thesignal light and the light pulse for fault localization use differentwavelengths, so that only the reflected and scattered light of the lightpulse for fault localization can enter the receiving-purpose opticalfiber 6b after traveling through the branch. The backwardly-scatteredlight of the signal light travels through the branch, but passes throughthe WDM coupler 2141 to come out from an unused node thereof. Suchwavelength selectivity is provided by the WDM coupler 2141.

In the fourth embodiment of FIG. 14, the remote amplifier 2D has thepost amplifier 20 and the pre-amplifier 21 which are integrated witheach other. Because of this integration, the post amplifier 20 and thepre-amplifier 21 are installed on the same position on the optical-fibercable 6. Depending on the characteristics of the signal light, however,there is a case in which an optimal position of the post amplifier 20and an optimal position of the pre-amplifier 21 are different. Ingeneral, the optimal position of the pre-amplifier 21 is more distantfrom the terminal station 1 than is the optimal position of the postamplifier 20.

FIGS. 16A and 16B are illustrative drawings showing variations of thefourth embodiment of the present invention. In these variations, theremote amplifier 2D is separated into two parts in order to place thepost amplifier 20 and the pre-amplifier 21 at optimum positions.

FIG. 16A shows a case in which the WDM coupler 2141 is provided at theinput of the fiber amplifier 211, and a branch is established betweenthe optical coupler 204 and the WDM coupler 2141. The remote amplifier2D is divided into a part 2D-1 and a part 2D-2 as shown in the figure.FIG. 16B shows a case in which the WDM coupler 2141 is provided at theoutput of the optical isolator 213, and a branch is established betweenthe optical coupler 204 and the WDM coupler 2141. The remote amplifier2D is divided into a part 2D-3 and a part 2D-4 as shown in the figure.

FIG. 17 is an illustrative drawing showing a fifth embodiment of arepeaterless transmission system according to the present invention. Inthe fifth embodiment, a localization-purpose transmission line 6f isprovided between the terminal station 1 and a remote amplifier 2E. Also,in the remote amplifier 2E, the optical coupler 214 is used forconnecting the localization-purpose transmission line 6f to thereceiving-purpose optical fiber 6b'.

In the normal operation mode, the localization-purpose transmission line6f is not used. During the time of fault localization, the OTDR 310 ofFIG. 3B is used as follows.

In the terminal station 1, the receiving unit 313 of the OTDR 310 isinstalled in place of the optical-signal receiving unit 111, and thesending unit 312 of the OTDR 310 is connected to a node 116 of thelocalization-purpose transmission line 6f. The sending unit 312transmits a light pulse for fault localization having the samewavelength λ₀ as the signal light. The light pulse for faultlocalization is sent to the remote amplifier 2E via thelocalization-purpose transmission line 6f, and is joined to thereceiving-purpose optical fiber 6b' by the optical coupler 214. Thefiber amplifier 211 is in an excitation state as in the normal operationmode, so that the reflected and scattered light coming back from thefault point 350 of the receiving-purpose optical fiber 6b' is amplifiedby the fiber amplifier 211. Then, the reflected and scattered lighttravels through the receiving-purpose optical fiber 6b to the terminalstation 1. The receiving unit 313 of the OTDR 310 detects the reflectedand scattered light.

In addition, the OTDR 300 of FIG. 3A is used as follows.

In the terminal station 1, the sending/receiving unit 302 of the OTDR300 is connected to the node 116 of the localization-purposetransmission line 6f. A light pulse for fault localization transmittedfrom the sending/receiving unit 302 reaches the fault point 350 via thelocalization-purpose optical fiber 6f, the optical coupler 214, and thereceiving-purpose optical fiber 6b'. The reflected and scattered lightcomes back to the sending/receiving unit 302 along the same route in areverse direction.

FIG. 18 is an illustrative drawing showing a sixth embodiment of arepeaterless transmission system according to the present invention. Thesixth embodiment is based on the configuration of the fifth embodimentof FIG. 17. In the sixth embodiment of FIG. 18, a remote amplifier 2Fincludes a WDM coupler 215, a fiber amplifier 216, a WDM coupler 217,and a WDM coupler 218 in addition to the elements provided in FIG. 17.The fiber amplifier 216 is placed between the localization-purposetransmission line 6f and the optical coupler 214 to amplify a lightpulse for fault localization. Also, bilateral excitation is applied tothe fiber amplifier 211 to enhance reception sensitivity for thereceiving signal light and the reflected and scattered light.

In the terminal station 1, a laser diode 113, a WDM coupler 114, and alaser diode 115 are provided in addition to the previously disclosedelements. The laser diode 113 is used for the rear-excitation of thefiber amplifier 216, and the laser diode 115 is used for thefront-excitation of the fiber amplifier 211. The WDM coupler 114 mixesthe pump light (wavelength λ₁) of the laser diode 112 and the pump light(wavelength λ₂) of the laser diode 113, and provides the mixed light tothe excitation-purpose optical fiber 6d.

In the remote amplifier 2F, the WDM coupler 218 splits the two pumplights sent through the excitation-purpose optical fiber 6d. The WDMcoupler 218 feeds the pump light of the wavelength λ₁ to the WDM coupler212, and feeds the pump light of the wavelength λ₂ to the WDM coupler217. The WDM coupler 215 receives the front-pump light (wavelength λ₁)sent through an excitation-purpose optical fiber 6g, and provides thefront-pump light to the input side of the fiber amplifier 211.

In the normal operation mode, the laser diode 113 of the terminalstation 1 is not used, so that the fiber amplifier 216 of the remoteamplifier 2F is not put in an excitation state. During the time of faultlocalization, the OTDR 310 of FIG. 3B is used as follows.

In the terminal station 1, the receiving unit 313 of the OTDR 310 isinstalled in place of the optical-signal receiving unit 111, and thesending unit 312 of the OTDR 310 is connected to the node 116 of thelocalization-purpose transmission line 6f. The sending unit 312transmits a light pulse for fault localization having the samewavelength λ₀ as the signal light. The light pulse for faultlocalization is sent to the remote amplifier 2F via thelocalization-purpose transmission line 6f, and is amplified by the fiberamplifier 216. Then, the light pulse for fault localization istransmitted to the receiving-purpose optical fiber 6b' after passingthrough the WDM coupler 217 and the optical coupler 214. The reflectedand scattered light coming back from the fault point 350 of thereceiving-purpose optical fiber 6b' passes through the optical coupler214 and the WDM coupler 215, and is amplified by the fiber amplifier211. Then, the reflected and scattered light travels through the WDMcoupler 212, the optical isolator 213, and the receiving-purpose opticalfiber 6b to finally reach the terminal station 1. The receiving unit 313of the OTDR 310 detects the reflected and scattered light.

The use of the remote amplifiers described above makes it possible tolocalize a fault point from the terminal stations even when the faultpoint is located beyond the remote amplifiers. Namely, the faultlocalization of the related art can be used when the fault point islocated between a terminal station and a remote amplifier, and the faultlocalization of the present invention can be used when the fault pointis located between the remote amplifiers.

FIG. 19 is an illustrative drawing showing ranges where a fault-pointcan be identified. When the fault-point is located on a sending-purposeoptical fiber within the 100-km range beyond a remote amplifier, any oneof the remote amplifiers 2A through 2D or its variation can be used fordetecting the fault point. In FIG. 19, this 100-km range is shown byhatches provided on a solid line representing the optical fiber. Whenthe fault-point is located on a receiving-purpose optical fiber withinthe 100-km range beyond a remote amplifier, one of the remote amplifiers2E and 2F or its variation can be used for detecting the fault point. InFIG. 19, this 100-km range is shown by crosses provided on a solid linerepresenting the optical fiber.

In this manner, according to the present invention, all ranges of theoptical-fiber cable can be covered by the fault localization in therepeaterless transmission system using remote amplifiers.

Further, the present invention is not limited to these embodiments, butvarious variations and modifications may be made without departing fromthe scope of the present invention.

What is claimed is:
 1. A device for receiving input signal light andpump light from a first remote site through a first optical fiber and asecond optical fiber, respectively, and for sending amplified signallight through a third optical fiber to a second remote site, said devicecomprising:amplifier means for amplifying said input signal light byusing said pump light to transmit said amplified signal light to saidthird optical fiber, said amplifier means having prevention means forpreventing light coming back through said third optical fiber fromentering said amplifier means; and path providing means for providing apath inside said device, said path leading reflection light comingthrough said third optical fiber to a connection point of said device,said connection point having optical connection with said first remotesite, wherein said reflection light is a light pulse for faultlocalization reflected at a fault point on said third optical fiber,said light pulse for fault localization sent to said third optical fiberfrom said first remote site via said device.
 2. The device as claimed inclaim 1, wherein said optical connection is provided by a fourth opticalfiber extending from said first remote site, and said path receives saidlight pulse for fault localization from said first remote site via saidfourth optical fiber.
 3. The device as claimed in claim 1, wherein saidoptical connection is provided by a fourth optical fiber extending fromsaid first remote site, and said amplifier means receives said lightpulse for fault localization through said first optical fiber.
 4. Thedevice as claimed in claim 1, wherein said optical connection isprovided by said second optical fiber, and said amplifier means receivessaid light pulse for fault localization through said first opticalfiber.
 5. The device as claimed in claim 1, wherein said opticalconnection is provided by said first optical fiber, and said amplifiermeans receives said light pulse for fault localization through saidfirst optical fiber.
 6. The device as claimed in claim 5, furthercomprising another amplifying means for amplifying said reflection lightalong said path.
 7. The device as claimed in claim 1, further comprisinganother amplifying means for receiving another pump light from saidfirst remote site and another input signal light from said second remotesite to send another amplified signal light to said first remote sitethrough a fifth optical fiber, wherein said optical connection isprovided by said fifth optical fiber, and said connection point is at aninput side of said another amplifying means.
 8. The device as claimed inclaim 1, further comprising another amplifying means for receivinganother pump light from said first remote site and another input signallight from said second remote site to send another amplified signallight to said first remote site through a fifth optical fiber, whereinsaid optical connection is provided by said fifth optical fiber, andsaid connection point is at an output side of said another amplifyingmeans.
 9. A device for receiving pump light from a first remote sitethrough a first optical fiber and input signal light from a secondremote site through a second optical fiber and for sending amplifiedsignal light to said first remote site through a third optical fiber,said device comprising:amplifier means for amplifying said input signallight by using said pump light to transmit said amplified signal lightto said third optical fiber, said amplifier means having preventionmeans for preventing light coming back through said third optical fiberfrom entering said amplifier means; and path providing means forproviding a path inside said device, said path receiving a light pulsefor fault localization sent from said first remote site at a connectionpoint and leading said light pulse for fault localization to said secondoptical fiber, said connection point having optical connection with saidfirst remote site, wherein reflection light of said light pulse forfault localization reflected at a fault point on said second opticalfiber is received by said amplifier means to be sent to said firstremote site via said third optical fiber.
 10. The device as claimed inclaim 9, wherein said optical connection is provided by a fourth opticalfiber.
 11. The device as claimed in claim 9, further comprises anotheramplifier means for amplifying said light pulse for fault localizationalong said path.
 12. A device for receiving first input signal light andfirst pump light from a first remote site to send first amplified signallight through a first optical fiber to a second remote site, and forreceiving second pump light from said first remote site and second inputsignal light through a second optical fiber from said second remote siteto send second amplified signal light through a third optical fiber tosaid first remote site, said device comprising:first amplifier means foramplifying said first input signal light by using said first pump lightto transmit said first amplified signal light to said first opticalfiber, said first amplifier means having prevention means for preventinglight coming through said first optical fiber from entering said firstamplifier means; first path providing means for providing a first pathinside said device for reflection light coming through said firstoptical fiber, said first path leading said reflection light to a fourthoptical fiber connected between said device and said first remote site;second amplifier means for amplifying said second input signal light byusing said second pump light to transmit said second amplified signallight to said third optical fiber, said second amplifier means havingprevention means for preventing light coming through said third opticalfiber from entering said second amplifier means; and second pathproviding means for providing a second path inside said device for alight pulse for fault localization sent from said first remote site,said second path leading said light pulse for fault localization to saidsecond optical fiber.
 13. A method of localizing a remote fault point ona first optical fiber from a local site in a transmission system where aremote amplifier receives input signal light from said local sitethrough a second optical fiber and pump light from said local sitethrough a third optical fiber to output amplified signal light to saidfirst optical fiber, said remote amplifier allowing only one-way passageof light from said local site to said first optical fiber; said methodcomprising the steps of:a) providing a path within said remoteamplifier, said path allowing passage of light from said first opticalfiber to a point which has optical connection with said local site; b)sending a fault-localization signal from said local site to said firstoptical fiber via said remote amplifier; and c) detecting, at said localsite, reflected light of said fault-localization signal reflected atsaid remote fault point on said first optical fiber, said reflectedlight coming back through said first optical fiber, said path, and saidoptical connection.
 14. The method as claimed in claim 13, furthercomprising a step of providing said optical connection via a fourthoptical fiber, wherein said step b) sends said fault-localization signalto said first optical fiber via said fourth optical fiber and said path.15. The method as claimed in claim 13, further comprising a step ofproviding said optical connection via a fourth optical fiber, whereinsaid step b) sends said fault-localization signal to said first opticalfiber via said second optical fiber and said remote amplifier.
 16. Themethod as claimed in claim 13, further comprising a step of providingsaid optical connection via said third optical fiber, wherein said stepb) sends said fault-localization signal to said first optical fiber viasaid third optical fiber and said path.
 17. The method as claimed inclaim 13, further comprising a step of providing said optical connectionvia said third optical fiber, wherein said step b) sends saidfault-localization signal to said first optical fiber via said secondoptical fiber and said remote amplifier.
 18. The method as claimed inclaim 13, further comprising a step of providing said optical connectionvia said second optical fiber, wherein said step b) sends saidfault-localization signal to said first optical fiber via said secondoptical fiber and one of said remote amplifier and said path.
 19. Themethod as claimed in claim 13, further comprising a step of providingsaid optical connection via a fifth optical fiber, said fifth opticalfiber being used by said remote amplifier when said remote amplifierreceives another input signal light from a remote site and sends anotheramplified signal light to said local site, wherein said step b) sendssaid fault-localization signal to said first optical fiber via saidsecond optical fiber and said remote amplifier.
 20. A method oflocalizing a remote fault point on a first optical fiber from a localsite in a transmission system where a remote amplifier receives inputsignal light through said first optical fiber and pump light from saidlocal site through a second optical fiber to send amplified signal lightto said local site through a third optical fiber, said remote amplifierallowing only one-way passage of light from said first optical fiber tosaid local site; said method comprising the steps of:a) providing a pathwithin said remote amplifier, said path allowing passage of light from apoint having optical connection with said local site to said firstoptical fiber; b) sending a fault-localization signal from said localsite to said first optical fiber via said optical connection and saidpath; and c) detecting, at said local site, reflected light of saidfault-localization signal reflected at said remote fault point on saidfirst optical fiber, said reflected light coming back through said firstoptical fiber, said remote amplifier, and said third optical fiber.